Numerous types of optical sensors sensitive to changes in refractive index have been characterized in the art. These include devices which operate by measuring optical energy internally reflected at an interface with a surrounding medium. Optical fibers may serve to direct light onto the interface and may also serve as the optical detectors themselves. Despite the availability of such sensors, there remains a need in the art for a rugged sensor using fiber-optics which is easy and inexpensive to produce, immune from electromagnetic interference and intrinsically safe in explosive environments.